The invention relates to a clamping device for the belt band of motor vehicle safety belts, the device having at least two relatively adjustable clamping elements for engagement with the belt band.
Such clamping devices have various designs and are also incorporated in self-locking belt band take-up reels so that, upon locking of the same due to the so-called film spool effect, withdrawal of the remaining belt band length of up to 100 mm is prevented and, up to maximum load, a belt band withdrawal of only about 30 mm, for example is permitted.
The motor vehicle-sensitive, lockable belt band take-up reel in such a conventional construction is movable relative to the downstream clamping device and is spring-biased away from the same. The clamping device consists of a stationary clamping element and a movable clamping element, namely, of a housing through which the belt band from the take-up reel extends and a shiftable clamping wedge in the housing next to the belt band. Upon locking of the take-up reel, a resulting tension in the belt band causes it to shift towards the clamping device against the action of its spring load. Consequently, the clamping wedge is likewise shifted in the belt band withdrawal direction to clamp the belt band in the housing. The flat clamping surface of the clamping wedge which faces one side of the belt band and the flat clamping surface of the housing which faces the other side of the belt band are each provided with a coating of resilient material having a high coefficient of friction (West German publication no. 3 242 783).
This known construction is relatively large, heavy and expensive. A further drawback is that the take-up reel is locked and the clamping device activated whenever the motor vehicle deceleration exceeds 0.45 g thereby adversely affecting the belt band since it is roughened during each of the frequent resulting clamping actions of the clamping device by the highly frictional coatings of the same. This gives rise to the danger that the belt band will be so severely damaged after the required 20,000 clamping actions that its strength is no longer adequate and the comfort of the safety belt is affected. The coating on the clamping surface of the housing has a particularly harmful effect because the belt band moves mainly with reference to this coating during each clamping procedure. This relative motion, and thus undesired withdrawal of the belt band, increase with increasing flexibility of the clamping device so that, in addition to their high coefficients of friction, the resilient design of the coatings of the clamping device also has a harmful effect. Such design further brings with it the danger that the clamping device will not release after a clamping procedure, particularly when the clamping wedge has a relatively steep wedge angle. It is true that damage to the belt band can be counteracted by allowing the clamping device to become operative only at relatively high motor vehicle decelerations such as occur in an accident. However, belt band withdrawal can then not be held to a minimum.
In a similar conventional construction, the clamping wedge cooperates with a movable clamping wall, and the clamping wedge and clamping wall are applied against the belt band in tandem with the clamping wedge moving towards the clamping wall. The two clamping surfaces for the belt band on the clamping wedge and the clamping wall are each provided with transverse ribs which are arranged such that the clamping surfaces can interengage one another in a complementary fashion to deform and clamp the belt band extending between them in a serpentine manner (U.S. Pat. No. 3,504,867).
This known construction is also relatively large, heavy and expensive. Another drawback is that the clamping wedge as well as the clamping wall have a relatively large mass and the transverse ribs of their belt band clamping surfaces are separated by a relatively large gap so that a correspondingly large mutual displacement is required for clamping of the belt band. Such displacement is retarded due to the inertia of the clamping wedge and the clamping wall so that it takes a relatively long time for the clamping wedge and the clamping wall to arrive at the clamping position with an unavoidable, correspondingly large belt band withdrawal. The belt band, which travels by the aggressive transverse ribs during this period, is very much affected by the transverse ribs as it is by the serpentine clamping which likewise has a negative effect on the strength of the belt band and the comfort of the safety belt. Thus, this construction, also, is suited for clamping of the belt band only once in an accident situation and with an accompanying large belt band withdrawal.
The European Patent Application No. 0 185 367 discloses a clamping device which includes a rigid clamping part having a relatively firm reaction surface with a low coefficient of friction and which thus clamps the belt band sufficiently gently on the corresponding side. A movable clamping element provided with gripping teeth grips the belt band on the opposite side. This is to occur synchronously, that is, with a speed of approach to the belt band which is equal to the speed of withdrawal of the belt.
In practice, however, uncontrollable effects due to the surroundings cannot be eliminated (belt band elongation, jerking upon belt withdrawal, manufacturing tolerances). Accordingly, on a microscopic level, relative movement of the belt band and the gripping teeth, which has an adverse effect on the belt band over time, occurs during penetration of the gripping teeth into the belt band and also upon removal.
The friction between the movable clamping element (clamping jaw) and the clamping lever is of great importance in achieving unblocking (release of the belt band after clamping) without problems. Only when relative movement can take place there under the existing clamping pressure is the return spring able to push back the clamping lever and the clamping part into the rest position.